Solder balls and the like conductive balls are often used in manufacturing the electronic components with bump; for example, a flip chip, a BGA (Ball Grid Array), etc. Suction head is a popular means for mounting the conductive balls on a chip, a substrate and other such workpieces.
Conductive balls kept in a vessel are picked up by a suction head having a number of suction holes provided in the bottom. Then the suction head moves to a place above a workpiece to have the conductive balls mounted on electrode of the workpiece. With this method, a number of balls can be carried to and mounted at once on a workpiece at a high operating efficiency.
In the above described mounting method using a suction head, however, the occurrence of a pickup error and a placing error with the conductive balls are not negligible. Namely, when a suction head is lowered to a dispensing vessel where a number of conductive balls are stored, the suction head does not always pick the conductive balls up rightly for sure, one conductive ball at each of the suction holes. For example, there may be a case where a plurality of conductive balls are sucked to by vacuum under one suction hole forming a chain of balls or a conductive ball is sticking on the bottom surface at a place other than the suction hole. The suction head goes up even if the sucking status remains as described above.
When a suction head is lowered on a workpiece to mount the conductive ball by releasing the sucking vacuum, it can not always be expected for sure that each and all of the conductive balls fall off the suction head to drop on the workpiece. Some of the conductive balls sometimes remain staying on the bottom surface of suction head even after it is lifted.
For detecting the existence of such redundant conductive ball sticking to the surface of suction head and such conductive ball still remaining on the suction head even after a mounting process is finished, there has been a detection system at work using a combination of a light sensor and a laser beam or the like light source. The above detection system recognizes the existence, or non-existence, of a conductive ball by making use of a variation in the amount of light received by the light sensor due to an interruption caused by a ball locating in the path of a light projected from the light source.
However, there are following problems in the above described detection system. The problems come from the machining errors during manufacture of a suction tool or the assembly errors thereof, and the level of precision during machining process for the suction hole. If there is a tilt with the bottom surface of a suction tool caused, for example, by a fabrication error, the bottom surface may not be precisely in parallel with the optical axis of a light sensor; in such a case, some difference may be caused in the interruption of projected light by a conductive ball depending on a place at which the conductive ball is being sucked to. If a wave form representing the amount of light received at the light sensor which exhibits a conductive ball under such a state is compared as it is with a threshold value for the purpose of judgement, the result of comparison may not always indicate correct status. It may misjudge the existence, or non existence, of the conductive ball. If there is a dispersion in the accuracy of machining for the tapered depth of the suction holes, then the height of the conductive balls being sucked to the suction holes may not be uniform. A judgement error may be induced likewise, also in this case.